The present invention relates to a process for producing potassium phosphate and more particularly to an ion exchange process wherein a metal phosphate salt solution is passed through a potassium loaded exchange resin so as to effect an exchange of potassium and the metal.
Since both potassium and phosphates are plant nutrients, potassium phosphate is a very effective fertilizer. Additionally, since it may be used with very little inert material present, it is easier to apply and may be transported relatively inexpensively.
Thus, in U.S. Pat. No. 4,008,307, an ion exchange process for producing potassium phosphate is disclosed which utilizes the potassium sulfate by-product from processes for the recovery of aluminum from alunite. More specifically, potassium ions from potassium sulfates are loaded onto a resin and then exchanged with phosphoric acid to yield the potassium phosphate.
Although effecting the desired exchange, the process is carried out in a single fixed bed exchange column and is therefore not very efficient since the process cannot be carried out continuously but rather, requires that the flow of materials be interrupted so that the resin can be regenerated. Additionally, fixed bed columns inherently require the presence of far greater amounts of resin than are actually being used at any given point in time due to the limited volume of the exchange zone, which proceeds downwardly as the upper layers of resin become spent. This requires that a far greater amount of resin be provided in the column than is actually being used in the exchange process at any one time, which translates into increased costs not only in terms of the additional quantities of resin required but also in terms of larger and more expensive equipment and higher processing costs. Another disadvantage inherent in fixed bed ion exchange systems for producing potassium phosphate stems from the fact that by the time the exchange zone nears the bottom of the column, the concentration gradient between the potassium bound to the resin and the exchange cation in the feed solution will have been substantially diminished thereby resulting in a concommitant reduction in the exchange efficiency.
Generally, when ion exchange processes are carried out in conjunction with high capacity exchange resins i.e., resins which become spent only after a large number of bed volumes of feed material have passed therethrough, it is not that detrimental that far greater amounts of resin are required since regeneration of the resin, as well as interruptions in the process to effect the same, will be infrequent. However, when the exchange resin is such that it does become loaded rapidly, then interruptions obviously take on greater significance and can cause a substantial decrease in the overall process efficiency. In similar fashion, since low capacity resins do require more frequent regeneration, it is especially important that the capacity which does exist, however limited, be used to the fullest extent possible.
Unfortunately, many of the exchange resins found to be most suited to potassium phosphate production fall into the low capacity category and become spent after only a few bed volumes of reactant have passed therethrough. Thus, resort has to be made to unduly large exchange columns having the aforementioned disadvantages in terms of excessive amounts of resin and diminishing concentration gradients as the feed materials proceed down the column.
Additionally, since the concentration of phosphate in the effluent being discharged from the first column in a fixed bed system has typically not yet reached commercially desirable levels, it is necessary to feed the effluent into yet another column. This amplifies the problem of excessive resin. Further, since the effluent will obviously contain depleted levels of feed materials, it will be necessary, in order to maintain a suitable concentration gradient, to fortify the effluent with additional feed materials. Such gives rise to extraordinary difficulties in terms of controlling the flow of materials in the process. More specifically, since the feed materials introduced into the one or more serially connected chambers as well as into the effluent streams would have to be frequently re-directed depending on whether or not the resin is being loaded, unloaded or treated in some other fashion, a very complicated valving system or the like would be required to monitor and regulate the flow of materials.
It will be readily appreciated therefore that it would be a difficult if not insurmountable task, in conjunction with a fixed bed system, to:
(i) carry out the process continuously, with the resin being regenerated with fresh potassium at least as quickly as it is spent by the exchange of the potassium with the phosphate salt feed material;
(ii) minimize the amount of resin in the exchange chambers, even though the resin is of the low capacity type; and
(iii) maintain a suitable concentration gradient between the phosphate salt feed and the potassium loaded onto the resin so as to ensure that the final product contains high enough levels of potassium phosphate.
Other processes for producing potassium phosphate have been devised, some of which are carried out by reacting two components such as a potassium salt and a metal phosphate directly i.e., without using an ion exchange column. However, these processes often require very expensive or difficult to obtain starting materials or alternatively, require very complicated processing conditions which render costs of such processes prohibitive.
Not surprisingly, therefore despite the advantages provided by potassium phosphates as fertilizers, they have not gained widespread acceptance due to the prohibitive costs of their production.